An apache2 security update has been released for Debian GNU/Linux 9 LTS to address several vulnerabilities.

DLA 2776-1: apache2 security update

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Debian LTS Advisory DLA-2776-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Sylvain Beucler
October 02, 2021   https://wiki.debian.org/LTS
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Package : apache2
Version : 2.4.25-3+deb9u11
CVE ID : CVE-2021-34798 CVE-2021-39275 CVE-2021-40438

Several vulnerabilities were discovered in the Apache HTTP server.
An attacker could send proxied requests to arbitrary servers, corrupt
memory in some setups involving third-party modules, and cause the
server to crash.

CVE-2021-34798

Malformed requests may cause the server to dereference
a NULL pointer.

CVE-2021-39275

ap_escape_quotes() may write beyond the end of a buffer when given
malicious input. No included modules pass untrusted data to these
functions, but third-party / external modules may.

CVE-2021-40438

A crafted request uri-path can cause mod_proxy to forward the
request to an origin server choosen by the remote user.

For Debian 9 stretch, these problems have been fixed in version
2.4.25-3+deb9u11.

We recommend that you upgrade your apache2 packages.

For the detailed security status of apache2 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/apache2

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS